What's an antenna if not something that can receive radio signals?

The same way an LED is not a solar panel. It will give you some voltage, but basically a rounding error above zero.

Antenna are about capturing energy over macro scale areas. This atom is measuring electromagnetic oscillation at a particular point in space. Technically you can recover a signal, but only a rounding error above the noise floor. It doesn't capture energy.

> What's an antenna if not something that can receive radio signals?

>> the instantaneous bandwidth is still too narrow to actually receive anything but narrowband carrier

so it is a "useless" antenna.